Pickup device structure within a device isolation region

ABSTRACT

A device includes a device isolation region formed into a semiconductor substrate, a doped pickup region formed into the device isolation region, a dummy gate structure that includes at least one structure that partially surrounds the doped pickup region, and a via connected to the doped pickup region.

PRIORITY INFORMATION

This Application is a continuation of U.S. Ser. No. 13/936,996 filedJul. 8, 2013, and entitled “Pickup Device Structure within a DeviceIsolation Region,” the disclosure of which is hereby incorporated byreference in its entirety.

BACKGROUND

Photo-sensitive devices are used in a variety of electronic devices. Forexample, an array of photo-sensitive devices can be used to form animage sensor array to be used in a digital camera. A photo-sensitivedevice typically includes an active region within a semiconductormaterial that transfers energy from photons into electrical energy.

Each cell within a photo-sensitive device array includes the mainphoto-sensitive region as well as some circuit components, such astransistors and resistors that are used to measure the electric currentproduced by the photo-sensitive device. It is important that thesecircuit components are isolated from the photo-sensitive region becausestray electric current can cause dark currents within thephoto-sensitive region. This adversely affects the light intensitymeasurements performed by the photo-sensitive region.

One way to isolate the device structures is to use shallow trenchisolation. Shallow trench isolation is a common technique used insemiconductor fabrication and involves the formation of a shallow trenchthat is then filled with a dielectric material. This technique, however,involves plasma etching which can damage the surface of the substrate.This can adversely affect the performance of the photo-sensitive array.

Another method of isolation is a technique referred to as deviceisolation. This technique involves the formation of a dopedsemiconductor material instead of a dielectric material. The dopedsemiconductor material is of a different concentration than the dopingconcentration of adjacent semiconductor materials, thus forming ajunction. However, this technique is less effective when isolatingheavily doped regions used for circuitry that is adjacent to thephoto-sensitive devices. Thus, it is desirable to find a method ofisolation that effectively protects the photo-sensitive devices withoutcausing damage to the surface of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a diagram showing an illustrative top view of an array ofphoto-sensitive devices, according to one example of principlesdescribed herein.

FIGS. 2A-2D are diagrams showing top and side views of a process forforming a pickup structure separated from photo-sensitive devices bydevice isolation, according to one example of principles describedherein.

FIG. 3A is a diagram showing an illustrative C-shaped gate structurewith a small gap, according to one example of principles describedherein.

FIG. 3B is a diagram showing an illustrative gate structure thatincludes four separate elongated structures, according to one example ofprinciples described herein.

FIG. 3C is a diagram showing an illustrative gate structure thatincludes a U-shaped structure and an elongated structure, according toone example of principles described herein.

FIG. 3D is a diagram showing an illustrative gate structure thatincludes two L-shaped structure, according to one example of principlesdescribed herein.

FIG. 4 is a flowchart showing an illustrative method for forming apickup structure separated by device isolation, according to one exampleof principles described herein.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the disclosure. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the performance of a first process before a second process in thedescription that follows may include embodiments in which the secondprocess is performed immediately after the first process, and may alsoinclude embodiments in which additional processes may be performedbetween the first and second processes. Various features may bearbitrarily drawn in different scales for the sake of simplicity andclarity. Furthermore, the formation of a first feature over or on asecond feature in the description that follows may include embodimentsin which the first and second features are formed in direct contact, andmay also include embodiments in which additional features may be formedbetween the first and second features, such that the first and secondfeatures may not be in direct contact.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. For example, if the device in the figures is turned over,elements described as being “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the exemplary term “below” can encompass both an orientation ofabove and below. The apparatus may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein may likewise be interpreted accordingly.

FIG. 1 is a diagram showing an illustrative top view of an array 100 ofphoto-sensitive devices. According to the present example, thephoto-sensitive array 100 includes a number of photo-sensitive devicessuch as photodiodes. Each photo-sensitive device 104 is associated witha set of circuitry 106. Additionally, the array 100 may include a numberof pickup devices 108. The pickup devices 108 are used to bias thedevice isolation regions, as will be described in further detail below.

Photodiodes 104 are commonly used in image sensor arrays to measure theintensity of light. Photodiodes 104 are often formed through use of aP-I-N junction. Such a junction includes an intrinsic semiconductorregion between a p-type doped region and an n-type doped region. Duringoperations, a reverse bias is typically applied to the photodiode 104. Areverse bias is where the p-type doped region is connected to a negativeterminal and a positive terminal is connected to the n-type region.Under such conditions, the photodiode 104 can be used to create eitheran electric current or a voltage. The strength of the electric currentor voltage is based on the intensity of light impinging on the activeregion of the photodiode 104.

To appropriately bias the photodiode 104 and measure any electriccurrent or voltage being created by the photodiode in response toimpinging photons, circuitry 106 is formed onto the substrate 102adjacent to each photodiode 104. The circuitry includes a variety ofcomponents including resistors and switching devices such as Metal OxideSemiconductor Field Effect Transistors (MOSFETs). Specifically, thecircuitry may include a reset transistor, a transfer transistor, asource follower transistor, and a row select transistor. Such switchingdevices also allow for addressing a specific photodiode 104 within thearray 100.

The electric current flowing through the switches and other circuitry106 associated with each photodiode 104 can cause issues if that currentleaks into the active region of the photodiode 104. Thus, it isimportant that the circuitry 106 is effectively isolated from the activeregion of the photodiode 104. Additionally, it is important that thedoped regions within the pickup devices are effectively isolated so thatelectric current does not flow into the active regions of thephotodiodes 104.

Isolation is often done using shallow trench isolation (STI), which isthe formation of a dielectric material into the semiconductor substrateadjacent to the circuitry to be isolated. However, the formation ofshallow trench isolation structures involves plasma etching. This plasmaetching process can have adverse effects on the surface of thesubstrate. Such adverse effects may impair the performance of the array100.

Another isolation technique is referred to as device isolation. Deviceisolation works by doping regions of a semiconductor substrate to definea device rather than etching a trench and forming a dielectric materialin that trench. The doped region effectively creates a junction with thebulk substrate material and prevents electrons from straying through.When forming the doped regions of a pickup device 108, the ionimplantation doping process can result in a doped region that is toolarge and extends too close to a photodiode 104. This can have anadverse effect on a nearby photodiode 104. Specifically it may causedark current and white pixel effects which will create errors in themeasurement of light impinging on the photodiode 104.

Pickup devices 108 may be used to connect the device isolation regionsto a voltage source. Application of a voltage to the device isolationregion can bias the device isolation region as desired. Biasing a deviceisolation region can allow the isolation to better isolate a particulardevice. In some cases, the entire substrate may be doped to form a largedevice isolation region. The only regions that are not doped and thusnot part of the device isolation region are where the photodiodes 104are to be formed and where the circuitry 106 associated with thosephotodiodes 104 are formed. Thus, the substrate may include a number ofpickup devices 108 positioned throughout the array. Each pickup device108 connects to a voltage source that is used to bias the deviceisolation region.

The pickup devices 108 serve as a connection between the deviceisolation region and a metal via that is used to connect the deviceisolation to a voltage source. It is generally not preferable todirectly connect the metal via to the device isolation region. A betterconnection is made if a highly doped region is formed into the deviceisolation region where the via is to be placed.

Because the space between photodiodes 104 is relatively small, thepickup devices 108 may have to be positioned between photodiodes 104 andwill thus be relatively close to the photodiodes 104. Thus, the ionimplantation doping process used to form the highly doped region intothe device isolation region may create a doped region that is too largeand extends beyond the device isolation region and come close or strayinto a photodiode region. As mentioned above, this creates adverseeffects such as dark current and white pixel effects. Thus, according toprinciples described herein, a gate-like structure is formed topartially encompass the region where the highly doped region is to beformed and thus limit the size of the highly doped region appropriately.

FIGS. 2A-2D are diagrams showing top and side views of a process forforming a pickup structure separated from photo-sensitive devices bydevice isolation. The left side of each figure illustrates a top view202. The right side of each figure illustrates a cross-sectional view204. The precise cross-section that is viewed is illustrated by thedotted line 220 shown with the top view 202.

FIG. 2A illustrates the formation of the device isolation region 208into the substrate 206. The substrate 206 may be made of a semiconductormaterial such as silicon. In some cases, the device isolation region 208may exist everywhere except for where components are to be formed. Forexample, the device isolation region 208 may be formed everywhere exceptfor where the photodiodes are placed and the circuitry associated withthose photodiodes.

As mentioned above, device isolation involves a doping of thesemiconductor substrate 206. In some cases, the substrate 206 itself maybe doped in a particular manner. In such cases, the device isolationregion may have the same type of doping but have a much higherconcentration of doping in order to create the junction. Variousdifferent device isolation techniques may be used in accordance withprinciples described herein.

The center portion of the top view 202 illustrates a region where thesubstrate is doped to form the device isolation region. The deviceisolation region 208 may include gaps where photodiodes can be formed.In the cross-sectional view 204, the middle region illustrates thedevice isolation region. Specifically, the middle region illustrateswhere the pickup device is to be formed.

FIG. 2B is a diagram showing the illustrative formation of photodiodes210 adjacent to the region where the pickup device is to be placed. Itis typically desirable to have an array with the photodiodes 210 asclose to each other as possible in order to obtain a high resolutionimage. Thus, the pickup devices are often formed directly between twoadjacent photodiodes 210.

FIG. 2C illustrates the formation of a dummy gate structure 212. Gatestructures are often used to form gate terminals for a transistor. Thegate terminal is a conductive material that is formed between a sourceregion and a drain region. The source and drain regions are typicallydoped semiconductor regions formed at opposing ends of the gatestructure. A transistor works by either allowing or blocking current toflow between the source terminal and the drain terminal. Depending onthe type of transistor, either a high or low signal will put thetransistor into an ON state, where electric current can flow between thesource and drain terminal. Conversely, an opposing signal applied at thegate will put the transistor into an OFF state, where electric currentis prevented from flowing between the source terminal and the drainterminal.

The dummy gate structure 212 may be formed using the same mask used toform the gate devices in the transistors of the circuitry associatedwith each photodiode. Thus, the dummy gate structure 212 that partiallyencompasses the region where the doped pickup region is to be formed, isformed at the same time, using the same processes, as the other gatestructures. The dummy gate structure 212 is referred to as a “dummy”gate structure because it does not perform the standard gate function,although it is made from the same material and same process.

The top view 202 illustrates a dummy gate structure 212 as a U-shapedstructure with the open portion of the U-shape pointing in a directionthat runs parallel to the photodiodes 210. The cross-sectional view 204illustrates the two extensions of the U-shaped dummy gate structure 212.As illustrated, the two extensions can block an ion implantation dopingprocess that is used to form the pickup region. Thus, the pickup regionis prevented from being so large that it extends too close to thephotodiodes 210. As will be described in further detail below, othershapes for gate structures 212 may be used in accordance with principlesdescribed herein. For example, the U-shaped dummy gate structure may beoriented differently. In some cases, multiple dummy gate structures 212may be used to fully encompass the pickup region except for small gaps.

The dummy gate structure 212 may be made of polysilicon. Polysilicon isa type of silicon that can conduct electric current and is thus suitablefor a real gate structure to be used for transistors. To make theprocess of forming the dummy gate structures 212 more efficient, theyare also made from the same material. The dummy gate structures 212,however, are not used not function as a gate device. The thickness ofthe dummy gate structure 212 may be based on design purposes.Specifically, the thickness of the dummy gate structure 212 may be suchthat the pickup regions to be formed are effectively confined within theintended area and do not extend too close to the photodiodes 210.

FIG. 2D is a diagram illustrating the formation of sidewall spacers 216on the gate structure 212 and formation of the pickup region 218.Sidewall spacers 216 may be made of a variety of materials includingsilicon oxide or silicon nitride. Sidewall spacers 216 are commonly usedin semiconductor fabrication processes. Sidewall spacers 216 are used togive some space between the gate structure 212 and the formation of adoped regions. In the case of transistors, adjacent doped regions may besource/drain regions. Here, in the case of a pickup device, the adjacentdoped regions are the doped pickup regions.

After the sidewall spacers 216 have been formed, the ion implantationdoping process can occur. This forms the pickup region 218 in thedesired location that is partially enclosed by the U-shaped dummy gatestructure 212. The dummy gate structure 212 blocks the ion implantationdoping process so that the highly doped pickup region is limited asdesired. While ion implantation is a relatively controlled process, itis not completely precise. Thus, by blocking the surrounding area withthe dummy gate structure 212, the risk of performing the ionimplantation process in an undesired location is reduced. Specifically,the pickup region is prevented from being formed so large so as to beformed too close to the photodiodes 210.

The top view 202 illustrates the formation of the pickup region 218within the dummy gate structure. Because the dummy gate structure 218partially encompasses the region where the pickup region is formed, thepickup region 218 is limited in size. This prevents the pickup regionfrom being larger than intended and extending into the photodiodes 210.

The cross-sectional view 204 illustrates the formation of the pickupregion 218 between the two extensions of the U-shaped dummy gatestructure 214. As illustrated, the sidewall spacer 216 displaces thesource/drain region 218 from being directly adjacent to the extensionsof the dummy gate structure 212.

In some cases, a lightly doped region, which is often referred to asLDD, can be formed before sidewall spacers 216 are put into place. Forexample, an LDD doping process is performed before the sidewall spacersare formed. The LDD process forms a doped region having a lower dopingconcentration than the final pickup region to be formed. Then, after thesidewall spacers have been formed, the main doping process that formsthe doped pickup region 218 is performed.

The cross-sectional view 204 also illustrates the formation of a via222. In some cases, the via 222 is formed after an interlayer dielectricmaterial 224 is formed over the layer of the dummy gate structure 212.The interlayer dielectric layer effectively isolates the components ofvarious layers from each other. Vias 222 are typically formed by etchinga hole into the interlayer dielectric layer 224 down to the underlyinglayer. In this case, the hole is formed down to the pickup region 218.Then, the hole is filled with a conductive material such as a metalmaterial. The via 222 ultimately connects the pickup region to a voltagesource. The voltage source is then used to bias the device isolationregion as desired.

In some cases, a via may connect to the dummy gate structure 212. Thisallows the dummy gate structure to be biased as well. The dummy gatestructure may be biased for various reasons to aid in the operation ofcircuitry within the photo-sensitive array.

FIG. 3A is a diagram showing an illustrative C-shaped gate structure 306with a small gap 302. According to the present example, the C-shapedgate structure 306 is similar to the U-shaped gate structure (e.g., 212,FIG. 2). The C-shaped gate structure 306 includes further extensions 304that point towards each other such that a small gap 302 is formed. Thus,the pickup region 218 is fully enclosed except for a small gap 304.Thus, the formation of the pickup region 218 is further limited. Thiscan help reduce the chances that electric current flowing through thepickup region 218 will leak through the device isolation region into thephotodiodes 210.

The gap 304 may be one of a variety of thicknesses. In one example, thethickness of the gap 204 is twice the thickness of the sidewall spacers216. Thus, the sidewall spacers 216 will effectively fill in the gap 304and the pickup region 218 will be completely encompassed. The ionimplantation doping process will thus be further limited and the pickupregion 218 will be kept to its desired size.

FIG. 3B is a diagram showing an illustrative gate structure thatincludes four separate elongated structures. According to the presentexample, the four elongated dummy gate structures 312 form a squareshape around the pickup region. Thus, there are four gaps 314. In thisexample, the gaps 314 are small enough so that the sidewall spacers 216fill in the gaps 314. In some examples, the different elongated dummygate structures 312 may be of different lengths. For example, the fourelongated dummy structures 312 may form a rectangular shape.

FIG. 3C is a diagram showing an illustrative gate structure thatincludes a U-shaped dummy gate structure 322 and an elongated dummy gatestructure 324. According to the present example, the elongated dummygate structure 324 is formed perpendicular to the open direction of theU-shaped dummy gate structure 322. Such a formation leaves two smallgaps 326. In this example, the gaps 326 are small enough so that thesidewall spacers 216 fill in the gaps 326.

FIG. 3D is a diagram showing an illustrative gate structure thatincludes two L-shaped dummy gate structures 332. According to thepresent example, the two L-shaped dummy structures 332 are formed withthe open ends facing each other to encompass the pickup region 218. Theposition of the L-shaped structures also leaves two gaps 334. In thisexample, the gaps 334 are small enough so that the sidewall spacers 216fill in the gaps 334.

The examples of dummy gate structure shapes as shown in FIG. 2 and FIGS.3A-3D are not an exhaustive list of possible shapes. Various othershaped dummy gate structures may be formed to partially encompass thepickup regions.

FIG. 4 is a flowchart showing an illustrative method for forming apickup structure separated by device isolation. According to certainillustrative examples, the method includes a step for forming 402 deviceisolation regions into a semiconductor substrate. The method furtherincludes a step for forming 404 photo-sensitive devices into gaps of thedevice isolation regions. The method further includes a step for forming406 a dummy gate structure over the substrate. The method furtherincludes a step for forming 408 a doped pickup region adjacent to thedummy gate structure. The method further includes a step for forming 410a via connected to the pickup region. The dummy gate structure includesone or more separate structures that partially encompass the dopedpickup region formed into the device isolation material.

According to certain illustrative examples, a device isolation regionformed into a semiconductor substrate, the device isolation regionhaving gaps for photo-sensitive devices, a dummy gate structure formedover the substrate, the dummy gate structure comprising at least onestructure that partially surrounds a doped pickup region formed into thedevice isolation region, and a via connected to the doped pickup region.

According to certain illustrative examples, a method for forming atransistor device to be used in association with a photo-sensitivedevice includes forming a device isolation region into a semiconductorsubstrate, forming photo-sensitive devices into gaps of the deviceisolation region, forming a dummy gate structure over the substrate,forming a doped pickup region adjacent to the dummy gate structure, andforming a via connected to the pickup region. The dummy gate structureincludes one or more separate structures that partially encompass thedoped pickup region formed into the device isolation region.

According to certain illustrative examples, a pickup device within anarray of photo-sensitive devices includes a device isolation regionformed into a semiconductor substrate, photo-sensitive devices beingisolated by the device isolation region, a doped pickup region formedinto the device isolation region, the doped pickup region being dopedwith a same type of material as the device isolation and having a higherdoping concentration than the device isolation region, a polysilicondummy gate structure formed over the substrate, the dummy gate structurecomprising at least one elongated structure that partially surrounds adoped pickup region formed into the device isolation region, and a viaconnected to the pickup region.

It is understood that various different combinations of the above-listedembodiments and steps can be used in various sequences or in parallel,and there is no particular step that is critical or required.Additionally, although the term “electrode” is used herein, it will berecognized that the term includes the concept of an “electrode contact.”Furthermore, features illustrated and discussed above with respect tosome embodiments can be combined with features illustrated and discussedabove with respect to other embodiments. Accordingly, all suchmodifications are intended to be included within the scope of thisinvention.

The foregoing has outlined features of several embodiments. Those ofordinary skill in the art should appreciate that they may readily usethe present disclosure as a basis for designing or modifying otherprocesses and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those of ordinary skill in the art should also realize that suchequivalent constructions do not depart from the spirit and scope of thepresent disclosure, and that they may make various changes,substitutions and alterations herein without departing from the spiritand scope of the present disclosure.

What is claimed is:
 1. A device comprising: a device isolation regionformed into a semiconductor substrate; a doped pickup region formed intothe device isolation region; a dummy gate structure that includes atleast one structure that partially surrounds the doped pickup region;and a via connected to the doped pickup region.
 2. The device of claim1, further comprising, gaps within the device isolation region.
 3. Thedevice of claim 2, further comprising, photo-sensitive devices formedwithin the gaps.
 4. The device of claim 1, wherein the dummy gatestructure is adjacent to a gap having a photo-sensitive device formedtherein.
 5. The device of claim 1, wherein the dummy gate structureforms a C-shape.
 6. The device of claim 5, wherein the gap in theC-shape dummy gate structure is twice the thickness of a spacer materialformed on the walls of the dummy structure.
 7. The device of claim 1,wherein the dummy gate structure forms a U-shape with an open end of theU-shape not facing towards a photo-sensitive device adjacent to thedummy gate structure.
 8. The device of claim 1, wherein the dummy gatestructure comprises four elongated structures that surround the dopedpickup region leaving four gaps.
 9. The device of claim 1, furthercomprising a sidewall spacer formed on the walls of the dummy gatestructure.
 10. The device of claim 1, wherein the pickup region is dopedwith the same type of dopant as the device isolation region and has ahigher doping concentration than the device isolation region.
 11. Thedevice of claim 1, wherein the dummy gate structure is biased.
 12. Thedevice of claim 1, wherein the dummy gate structure is made ofpolysilicon.
 13. A method comprising: forming a device isolation regioninto a semiconductor substrate; forming a dummy gate structure over thesubstrate; forming a doped pickup region adjacent to the dummy gatestructure; and forming a via connected to the pickup region; wherein,the dummy gate structure includes one or more separate structures thatpartially encompass the doped pickup region formed into the deviceisolation region.
 14. The method of claim 13, wherein the dummy gatestructure forms a C-shape.
 15. The method of claim 14, wherein the gapin the C-shape dummy gate structure is twice the thickness of a spacermaterial formed on the walls of the dummy gate structure.
 16. The methodof claim 13, wherein the dummy gate structure forms a U-shape with anopen end of the U-shape not facing towards a photo-sensitive deviceadjacent to the dummy gate structure.
 17. The method of claim 13,wherein the dummy gate structure comprises four elongated structuresthat surround the pickup region leaving four gaps.
 18. The method ofclaim 13, wherein the dummy gate structure comprises two L-shapedstructures with two small gaps, the gaps having a width that is abouttwice the thickness of a spacer material formed on walls of the dummygate structure.
 19. The method of claim 13, wherein the pickup region isdoped with the same type of dopant as the device isolation region andhas a higher doping concentration than the device isolation region. 20.A pickup device comprising: a device isolation region formed into asemiconductor substrate; a doped pickup region formed into the deviceisolation region, the doped pickup region being doped with a same typeof material as the device isolation and having a higher dopingconcentration than the device isolation region; a polysilicon dummy gatestructure formed over the substrate, the dummy gate structure comprisingan elongated structure that at least partially surrounds the dopedpickup region formed into the device isolation region; and a viaconnected to the pickup region.